Gelled polymer electrolyte lithium secondary cell

ABSTRACT

The battery of this invention includes a positive electrode including a gelled polymeric electrolyte (A) and using spinel type lithium manganese oxide as an active material; a negative electrode; a gelled polymeric electrolyte (B) in the shape of a film or sheet also serving as a separator, and both the gelled polymeric electrolyte (A) and the gelled polymeric electrolyte (B) are made from a polymer of poly(alkylene oxide) series impregnated with a liquid electrolyte. Since the battery includes the positive electrode using the specific gelled polymeric electrolyte (A), a contact area between the positive electrode active material and the gelled polymeric electrolyte is large, so as to attain large initial discharge capacity (at high rate discharge in particular). Also, since the battery includes the specific gelled polymeric electrolyte (B) as the electrolyte, manganese included in the spinel type lithium manganese oxide is minimally eluted, and hence, the discharge capacity is minimally degraded during charge-discharge cycles due to elution of manganese, resulting in attaining good charge-discharge cycle performance.

TECHNICAL FIELD

The present invention relates to a lithium secondary battery includingspinel type lithium manganese oxide as a positive electrode activematerial and gelled polymeric electrolyte as an ion conducting medium,and more particularly, it relates to improvement of a positive electrodefor the purpose of providing a lithium secondary battery using a gelledpolymeric electrolyte with large initial discharge capacity and goodcharge-discharge cycle performance.

BACKGROUND ART

As an ion conducting medium (electrolyte) of a lithium secondarybattery, a liquid electrolyte (electrolytic solution) has beenconventionally used because of its good ionic conductivity although ithas problems of leakage and elution of an electrode material.

When a liquid electrolyte is used as an ion conducting medium in alithium secondary battery using lithium manganese oxide as a positiveelectrode active material, however, manganese included in the lithiummanganese oxide is gradually eluted into the liquid electrolyte,resulting in causing a problem that the discharge capacity is degradedin a small number of charge-discharge cycles.

When a solid electrolyte (such as a film and a foil) is used instead ofa liquid electrolyte, the degradation of the discharge capacity due tothe elution of manganese into the electrolyte can be avoided. The ionicconductivity of the solid electrolyte is, however, generally lower thanthat of the liquid electrolyte, and a contact area between theelectrolyte and the electrode is so small that the electric resistance(interface resistance) on the interface between the electrolyte and theelectrode is large. Therefore, the discharge capacity, at high ratedischarge in particular, is degraded.

Accordingly, as an ion conducting medium for improving the disadvantagesof and making the best use of the advantages of the liquid electrolyteand the solid electrolyte, a gelled electrolyte, particularly a gelledpolymeric electrolyte that can be easily formed into a thin film and isinexpensive, has been recently proposed. A gelled polymeric electrolyteis a gelled substance obtained by impregnating a liquid electrolyteincluding a solute (electrolytic salt) and a solvent into a matrix of apolymer (resin). Since a gelled polymeric electrolyte includes a liquidelectrolyte, it has higher ionic conductivity than a solid electrolyte,and since the liquid electrolyte is fixed through gelation within thematrix of the gelled polymeric electrolyte, manganese is minimallyeluted into the liquid electrolyte.

When a gelled polymeric electrolyte is used, however, a contact areabetween the electrode and the electrolyte is smaller than in using aliquid electrolyte. Therefore, the electric resistance (interfaceresistance) on the interface between the electrode and the electrolyteis large as in using a solid electrolyte. As a result, the dischargecapacity, at high rate discharge in particular, is degraded.

Accordingly, an object of the invention is providing a lithium secondarybattery using a gelled polymeric electrolyte with large initialdischarge capacity and good charge-discharge cycle performance.

DISCLOSURE OF INVENTION

The lithium secondary battery using a gelled polymeric electrolyte ofthis invention (present battery) comprises a positive electrodeincluding a gelled polymeric electrolyte (A) and using spinel typelithium manganese oxide as an active material; a negative electrode; anda gelled polymeric electrolyte (B) in the shape of a film or sheet alsoserving as a separator, and the gelled polymeric electrolyte (A) and thegelled polymeric electrolyte (B) are made from a polymer ofpoly(alkylene oxide) series impregnated with a liquid electrolyte.

Both the gelled polymeric electrolyte (A) included in the positiveelectrode and the gelled polymeric electrolyte (B) in the shape of afilm or sheet also serving as a separator are the polymers ofpoly(alkylene oxide) series impregnated with a liquid electrolyte.

Examples of the polymer of poly(alkylene oxide) series are poly(ethyleneoxide), poly(propylene oxide), a block copolymer of poly(ethylene oxide)and polystyrene, a block copolymer of poly(ethylene oxide) andpolypropylene oxide, polyetherimide, polyethersulfone, polysiloxane andpolysulfone. From the viewpoint of the charge-discharge cycleperformance, the block copolymer of poly(ethylene oxide) and polystyreneis particularly preferred. In particular, the polymer of poly(alkyleneoxide) series used for the gelled polymeric electrolyte (B) preferablyhas high mechanical strength and a large molecular weight. When, forexample, poly(ethylene oxide) is used, the number average molecularweight Mn is preferably approximately two million through eight million.

Examples of the electrolyte used for impregnating the polymer ofpoly(alkylene oxide) series are LiClO₄, LiCF₃SO₃, LiPF₆, LiBF₄, LiSbF₆,LiAsF₆, LiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) (wherein m and nindependently indicate an integer ranging between 1 and 5), andLiC(C_(p)F_(2p+1)SO₂)(C_(q)F_(2q+1)SO₂)(C_(r)F_(2r+1)SO₂) (wherein p, qand r independently indicate an integer ranging between 1 and 5).Examples of the solvent are ethylene carbonate, propylene carbonate,butylene carbonate, vinylene carbonate, dimethyl carbonate, diethylcarbonate, methylethyl carbonate, γ-butyrolactone, sulfolane,1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-ethoxymethoxyethane,tetrahydrofuran, 2-methyl-1,3-dioxolane, 4-methyl-1,3-dioxolane,dimethyl ether, diethyl ether, ethyl acetate and methyl propionate. Theliquid electrolyte preferably includesLiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) (wherein m and n independentlyindicate an integer ranging between 1 and 5) and/orLiC(C_(p)F_(2p+1)SO₂)(C_(q)F_(2q+1)SO₂)(C_(r)F_(2r+1)SO₂) (wherein p, qand r independently indicate an integer ranging between 1 and 5) in aconcentration of 0.1 through 2.0 mol/liter because the elution ofmanganese can be thus effectively suppressed during charge-dischargecycles. When another solute is used together withLiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) (wherein m and n independentlyindicate an integer ranging between 1 and 5) and/orLiC(C_(p)F_(2p+1)SO₂)(C_(q)F_(2q+1)SO₂)(C_(r)F_(2r+1)SO₂) (wherein p, qand r independently indicate an integer ranging between 1 and 5), theconcentration of the solutes in the liquid electrolyte is preferablylower than 2.0 mol/liter.

In the case where the gelled polymeric electrolyte (A) and the gelledpolymeric electrolyte (B) are made from the same material, a film of apolymer of poly(alkylene oxide) series is formed on a positive electrodeby a casting method or the like, and part of the cast polymer ofpoly(alkylene oxide) series is allowed to be included in the positiveelectrode at the same time. Subsequently, the polymer of poly(alkyleneoxide) series is impregnated with the same liquid electrolyte. In thismanner, the film-like gelled polymeric electrolyte (B) also serving asthe separator and the positive electrode including the gelled polymericelectrolyte (A) are preferably integrally fabricated because thefabrication can be thus eased and the contact resistance between thegelled polymeric electrolyte also serving as the separator and thepositive electrode can be lowered.

The spinel type lithium manganese oxide used as the active material ofthe positive electrode is lithium manganese oxide having a spinelstructure belonging to the cubic system. A specific example of thespinel type lithium manganese oxide is LiM_(x)Mn_(2−x)O₄ (wherein M isat least one element selected from the group consisting of Ni, Al, Mg,Fe and Co; and 0≦x≦0.5).

Examples of the material for the negative electrode are a substancecapable of electrochemically occluding and discharging lithium ions andmetallic lithium. Examples of the substance capable of electrochemicallyoccluding and discharging lithium ions are a carbon material such asgraphite (natural graphite and artificial graphite), coke and an organicbaked substance; lithium alloy such as lithium—aluminum alloy, lithium-magnesium alloy, lithium—indium alloy, lithium—tin alloy,lithium—thallium alloy, lithium—lead alloy and lithium—bismuth alloy;and a metal oxide or metal sulfide including one of or two or more oftin, titanium, iron, molybdenum, niobium, vanadium and zinc.

Since the present battery uses a positive electrode including a specificgelled polymeric electrolyte (A), the contact area between the positiveelectrode active material and the gelled polymeric electrolyte is large.Accordingly, the present battery can attain large initial dischargecapacity (at high rate discharge in particular). Also, since the presentbattery uses a specific gelled polymeric electrolyte (B) as theelectrolyte, manganese included in spinel type lithium manganese oxideis minimally eluted, which can reduce the degradation of the dischargecapacity derived from elution of manganese during charge-dischargecycles. Accordingly, the present battery can exhibit goodcharge-discharge cycle performance.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a sectional view of a lithium secondary battery using a gelledpolymeric electrolyte fabricated in an embodiment.

PREFERRED EMBODIMENTS

The present invention will now be described in detail on the basis ofpreferred embodiments thereof, and it is noted that the invention is notlimited to the following embodiments but can be practiced withappropriate modification without departing from the scope of theinvention.

Embodiment 1

Preparation of Positive Electrode

Lithium nitrate (LiN₃), nickel nitrate (Ni(NO₃)₂) and manganese acetate(Mn(CH₃COO)₂) were mixed in a molar ratio of 1:0.4:1.6, and the mixturewas baked at 600° C. for 24 hours in the air, thereby obtainingLiNi_(0.4)Mn_(1.6)O₄. Then, the LiNi_(0.4)Mn_(1.6)O₄ was crushed with ajet mill, so as to prepare a spinel type lithium manganese oxide powderwith a median diameter of 10 μm. The spinel type lithium manganese oxidepowder, a carbon powder serving as a conductive agent and apoly(vinylidene fluoride) powder serving as a binder were mixed in aweight ratio of 85:10:5, and the mixture was kneaded with NMP(N-methyl-2-pyrrolidone) to give a paste. The paste was applied on apositive electrode collector (stainless steel plate) by a doctor blademethod (to attain a thickness of 80 μm after drying), and the resultantwas heated at 130° C., thereby preparing a positive electrode (porouselectrode) in the shape of a disk with a diameter of 10 mm.

Preparation of Gelled Polymeric Electrolyte also Serving as Separator

Poly(ethylene glycol) ethyl ether acrylate (with a number averagemolecular weight Mn of 360; CH₂═CH—COO—(CH₂—CH₂—O)n—CH₂—CH₃) and aliquid electrolyte obtained by dissolving 1 M (mol/liter) of LiClO₄ in amixed solvent including ethylene carbonate and dimethyl carbonate in avolume ratio of 2:3 were mixed in a weight ratio of 1:1. The mixture wasapplied on the positive electrode into a thickness of 25 μm, andirradiated with electron beams with a beam irradiation apparatus withelectron curtain system (having output power of 200 kV, exposure of 2Mrad and a travel speed of an irradiated target of 1 m/min.), therebypolymerizing the poly(ethylene glycol) ethyl ether acrylate. Thus, agelled polymeric electrolyte film also serving as a separator was formedon one face of the positive electrode. It was confirmed throughobservation with a scanning electron microscope (SEM) that the gelledpolymeric electrolyte had entered the inside of the positive electrode.

Preparation of Negative Electrode

A graphite powder with an average particle size of 10 μm serving as alithium ion occluding agent and poly(vinylidene fluoride) serving as abinder were mixed in a weight ratio of 95:5, and the mixture was kneadedwith NMP to give a paste. The paste was applied on a negative electrodecollector (stainless steel plate) by the doctor blade method (to attaina thickness of 70 μm after drying), and the resultant was heated at 130°C., thereby preparing a negative electrode in the shape of a disk with adiameter of 10 mm.

Fabrication of Lithium Secondary Battery using Gelled PolymericElectrolyte

The negative electrode was placed on the gelled polymeric electrolytefilm formed on one face of the positive electrode and the resultant washoused in a battery can, thereby fabricating a flat lithium secondarybattery (present battery) A1 using the gelled polymeric electrolyte. Thecapacity ratio between the positive electrode and the negative electrodewas set to 1:1.1, so as to control the battery capacity by the positiveelectrode capacity. In every battery described below, the capacity ratiobetween the positive electrode and the negative electrode was also setto 1:1.1. FIG. 1 is a sectional view of the lithium secondary battery A1using the gelled polymeric electrolyte, and the battery A1 of FIG. 1comprises a positive electrode 1, a negative electrode 2, a gelledpolymeric electrolyte film 3 for separating these electrodes, a positiveelectrode can 4, a negative electrode can 5, a positive electrodecollector 6, a negative electrode collector 7, an insulating packing 8and the like. The positive electrode 1 and the negative electrode 2opposing each other with the gelled polymeric electrolyte film 3impregnated with a liquid electrolyte sandwiched therebetween are housedin the battery can formed by the positive electrode can 4 and thenegative electrode can 5. The positive electrode 1 is connected to thepositive electrode can 4 through the positive electrode collector 6,and, the negative electrode 2 is connected to the negative electrode can5 through the negative electrode collector 7, so that chemical energygenerated within the battery can be taken out as electric energy.

Embodiment 2

Lithium nitrate, aluminum hydroxide (Al(OH)₃) and manganese acetate weremixed in a molar ratio of 1:0.4:1.6, and the mixture was baked at 800°C. for 24 hours in the air, thereby obtaining LiAl_(0.4)Mn_(1.6)O₄. TheLiAl_(0.4)Mn_(0.6)O₄ was crushed with a jet mill to give a spinel typelithium manganese oxide powder with a median diameter of 10 μm. Thespinel type lithium manganese oxide powder, a carbon powder serving as aconductive agent and a poly(vinylidene fluoride) powder serving as abinder were mixed in a weight ratio of 85:10:5. The mixture was kneadedwith NMP to give a paste. The paste was applied on a positive electrodecollector (stainless steel plate) by the doctor blade method (to attaina thickness of 80 μm after drying), and the resultant was heated at 130°C. Thus, a positive electrode (porous electrode) in the shape of a diskwith a diameter of 10 mm was prepared.

Next, poly(ethylene glycol) ethyl ether acrylate (with a number averagemolecular weight Mn of 360; CH₂═CH—COO—(CH₂—CH₂—O)n—CH₂—CH₃) and aliquid electrolyte obtained by dissolving 1 M (mol/liter) of LiClO₄ in amixed solvent including ethylene carbonate and dimethyl carbonate in avolume ratio of 2:3 were mixed in a weight ratio of 1:1. The resultantwas applied on the positive electrode into a thickness of 25 μm, andirradiated with electron beams with a beam irradiation apparatus withelectron curtain system (having output power of 200 kV, exposure of 2Mrad and a travel speed of an irradiated target of 1 m/min.), therebypolymerizing the poly(ethylene glycol) ethyl ether acrylate. Thus, agelled polymeric electrolyte film also serving as a separator was formedon one face of the positive electrode. It was confirmed throughobservation with a scanning electron microscope (SEM) that the gelledpolymeric electrolyte had entered the inside of the positive electrode.

Then, a negative electrode (the same as that prepared in Embodiment 1)was placed on the gelled polymeric electrolyte film formed on one faceof the positive electrode and the resultant was housed in a battery can.Thus, a flat lithium secondary battery (present battery) A2 using thegelled polymeric electrolyte was fabricated.

Embodiment 3

Lithium nitrate, magnesium acetate (Mg(CH₃COO)₂) and manganese acetatewere mixed in a molar ratio of 1:0.4:1.6, and the mixture was baked at700° C. for 24 hours in the air, thereby obtaining LiMg_(0.4)Mn_(1.6)O₄.The LiMg_(0.4)Mn_(0.6)O₄ was crushed with a jet mill to give a spineltype lithium manganese oxide powder with a median diameter of 10 μm. Thespinel type lithium manganese oxide powder, a carbon powder serving as aconductive agent and a poly(vinylidene fluoride) powder serving as abinder were mixed in a weight ratio of 85:10:5. The mixture was kneadedwith NMP to give a paste. The paste was applied on a positive electrodecollector (stainless steel plate) by the doctor blade method (to attaina thickness of 80 μm after drying), and the resultant was heated at 130°C. Thus, a positive electrode (porous electrode) in the shape of a diskwith a diameter of 10 mm was prepared.

Next, poly(ethylene glycol) ethyl ether acrylate (with a number averagemolecular weight Mn of 360; CH₂═CH—COO—(CH₂—CH₂—O)n—CH₂—CH₃) and aliquid electrolyte obtained by dissolving 1 M (mol/liter) of LiClO₄ in amixed solvent including ethylene carbonate and dimethyl carbonate in avolume ratio of 2:3 were mixed in a weight ratio of 1:1. The resultantwas applied on the positive electrode into a thickness of 25 μm, andirradiated with electron beams with a beam irradiation apparatus withelectron curtain system (having output power of 200 kV, exposure of 2Mrad and a travel speed of an irradiated target of 1 m/min.), therebypolymerizing the poly(ethylene glycol) ethyl ether acrylate. Thus, agelled polymeric electrolyte film also serving as a separator was formedon one face of the positive electrode. It was confirmed throughobservation with a scanning electron microscope (SEM) that the gelledpolymeric electrolyte had entered the inside of the positive electrode.

Then, a negative electrode (the same as that prepared in Embodiment 1)was placed on the gelled polymeric electrolyte film formed on one faceof the positive electrode and the resultant was housed in a battery can.Thus, a flat lithium secondary battery (present battery) A3 using thegelled polymeric electrolyte was fabricated.

Embodiment 4

Lithium nitrate, ferric nitrate (Fe(NO₃)₃) and manganese acetate weremixed in a molar ratio of 1:0.4:1.6, and the mixture was baked at 700°C. for 24 hours in the air, thereby obtaining LiFe_(0.4)Mn_(1.6)O₄. TheLiFe_(0.4)Mn_(0.6)O₄ was crushed with a jet mill to give a spinel typelithium manganese oxide powder with a median diameter of 10 μm. Thespinel type lithium manganese oxide powder, a carbon powder serving as aconductive agent and a poly(vinylidene fluoride) powder serving as abinder were mixed in a weight ratio of 85:10:5. The mixture was kneadedwith NMP to give a paste. The paste was applied on a positive electrodecollector (stainless steel plate) by the doctor blade method (to attaina thickness of 80 μm after drying), and the resultant was heated at 130°C. Thus, a positive electrode (porous electrode) in the shape of a diskwith a diameter of 10 mm was prepared. Next, poly(ethylene glycol) ethylether acrylate (with a number average molecular weight Mn of 360;CH₂═CH—COO—(CH₂—CH₂—O)n—CH₂—CH₃) and a liquid electrolyte obtained bydissolving 1 M (mol/liter) of LiClO₄ in a mixed solvent includingethylene carbonate and dimethyl carbonate in a volume ratio of 2:3 weremixed in a weight ratio of 1:1. The resultant was applied on thepositive electrode into a thickness of 25 μm, and irradiated withelectron beams with a beam irradiation apparatus with electron curtainsystem (having output power of 200 kV, exposure of 2 Mrad and a travelspeed of an irradiated target of 1 m/min.), thereby polymerizing thepoly(ethylene glycol) ethyl ether acrylate. Thus, a gelled polymericelectrolyte film also serving as a separator was formed on one face ofthe positive electrode. It was confirmed through observation with ascanning electron microscope (SEM) that the gelled polymeric electrolytehad entered the inside of the positive electrode.

Then, a negative electrode (the same as that prepared in Embodiment 1)was placed on the gelled polymeric electrolyte film formed on one faceof the positive electrode and the resultant was housed in a battery can.Thus, a flat lithium secondary battery (present battery) A4 using thegelled polymeric electrolyte was fabricated.

Embodiment 5

Lithium nitrate, cobalt acetate (Co(CH₃COO)₂) and manganese acetate weremixed in a molar ratio of 1:0.4:1.6, and the mixture was baked at 700°C. for 24 hours in the air, thereby obtaining LiCo_(0.4)Mn_(1.6)O₄. TheLiCo_(0.4)Mn_(0.6)O₄ was crushed with a jet mill to give a spinel typelithium manganese oxide powder with a median diameter of 10 μm. Thespinel type lithium manganese oxide powder, a carbon powder serving as aconductive agent and a poly(vinylidene fluoride) powder serving as abinder were mixed in a weight ratio of 85:10:5. The mixture was kneadedwith NMP to give a paste. The paste was applied on a positive electrodecollector (stainless steel plate) by the doctor blade method (to attaina thickness of 80 μm after drying), and the resultant was heated at 130°C. Thus, a positive electrode (porous electrode) in the shape of a diskwith a diameter of 10 mm was prepared. Next, poly(ethylene glycol) ethylether acrylate (with a number average molecular weight Mn of 360;CH₂═CH—COO—(CH₂—CH₂—O)n—CH₂—CH₃) and a liquid electrolyte obtained bydissolving 1 M (mol/liter) of LiClO₄ in a mixed solvent includingethylene carbonate and dimethyl carbonate in a volume ratio of 2:3 weremixed in a weight ratio of 1:1. The resultant was applied on thepositive electrode into a thickness of 25 μm, and irradiated withelectron beams with a beam irradiation apparatus with electron curtainsystem (having output power of 200 kV, exposure of 2 Mrad and a travelspeed of an irradiated target of 1 m/min.), thereby polymerizing thepoly(ethylene glycol) ethyl ether acrylate. Thus, a gelled polymericelectrolyte film also serving as a separator was formed on one face ofthe positive electrode. It was confirmed through observation with ascanning electron microscope (SEM) that the gelled polymeric electrolytehad entered the inside of the positive electrode.

Then, a negative electrode (the same as that prepared in Embodiment 1)was placed on the gelled polymeric electrolyte film formed on one faceof the positive electrode and the resultant was housed in a battery can.Thus, a flat lithium secondary battery (present battery) A5 using thegelled polymeric electrolyte was fabricated.

Embodiment 6

A block copolymer of poly(ethylene oxide) and polystyrene (with acopolymerization ratio of 1:1 and a number average molecular weight Mnof approximately 300,000) was dissolved in NMP to give a solution (witha solid content of 20 wt %). The solution was applied on one face of apositive electrode the same as that prepared in Embodiment 1 into athickness of 25 μm, thereby forming a block copolymer film ofpoly(ethylene oxide) and polystyrene. This positive electrode wasimmersed in a liquid electrolyte obtained by dissolving 1 M of LiClO₄ ina mixed solvent including ethylene carbonate and dimethyl carbonate in avolume ratio of 2:3, so as to impregnate the block copolymer ofpoly(ethylene oxide) and polystyrene. with the liquid electrolyte. Thus,a gelled polymeric electrolyte film also serving as a separator wasformed on one face of the positive electrode. It was confirmed throughobservation with a scanning electron microscope that the gelledpolymeric electrolyte had entered the inside of the positive electrode.Next, a negative electrode (the same as that prepared in Embodiment 1)was placed on the gelled polymeric electrolyte film formed on one faceof the positive electrode and the resultant was housed in a battery can.Thus, a flat lithium secondary battery (present battery) A6 using thegelled polymeric electrolyte was fabricated.

COMPARATIVE EXAMPLE 1

A flat lithium secondary battery (comparative battery) B1 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment1 except for the following: Instead of the gelled polymeric electrolytefilm also serving as a separator, a liquid electrolyte obtained bydissolving 1 M of LiClO₄ in a mixed solvent including ethylene carbonateand dimethyl carbonate in a volume ratio of 2:3 and a polyethylenemicroporous film (separator) with a thickness of 25 μm were used, andthe gelled polymeric electrolyte was not included in the positiveelectrode.

COMPARATIVE EXAMPLE 2

Poly(vinylidene fluoride) (with a number average molecular weight Mn ofapproximately 110,000) was dissolved in acetone to give a solution (witha solid content of 10 wt %). The solution was applied on one face of apositive electrode the same as that prepared in Embodiment 1 into athickness of 25 μm, thereby forming a poly(vinylidene fluoride) film.The positive electrode was immersed in a liquid electrolyte obtained bydissolving 1 M of LiClO₄ in a mixed solvent including ethylene carbonateand dimethyl carbonate in a volume ratio of 2:3, so as to impregnate thepoly(vinylidene fluoride) with the liquid electrolyte. Thus, a gelledpolymeric electrolyte film also serving as a separator was formed on oneface of the positive electrode. It was confirmed through observationwith a scanning electron microscope that the gelled polymericelectrolyte had entered the inside of the positive electrode. Next, anegative electrode (the same as that prepared in Embodiment 1) wasplaced on the gelled polymeric electrolyte film formed on one face ofthe positive electrode and the resultant was housed in a battery can.Thus, a flat lithium secondary battery (comparative battery) B2 usingthe gelled polymeric electrolyte was prepared.

COMPARATIVE EXAMPLE 3

A block copolymer of poly(ethylene oxide) and polystyrene (with acopolymerization ratio of 1:1 and a number average molecular weight Mnof approximately 300,000) was dissolved in NMP to give a solution (witha solid content of 20 wt %). The solution was formed by the castingmethod into a block copolymer film of poly(ethylene oxide) andpolystyrene with a thickness of 25 μm. Then, the film was immersed in aliquid electrolyte obtained by dissolving 1 M of LiClO₄ in a mixedsolvent including ethylene carbonate and dimethyl carbonate in a volumeratio of 2:3, so as to impregnate the block copolymer of poly(ethyleneoxide) and polystyrene with the liquid electrolyte by 100 wt %. Thus, agelled polymeric electrolyte film also serving as a separator wasformed. Next, the gelled polymeric electrolyte film was sandwichedbetween a positive electrode and a negative electrode the same as thoseprepared in Embodiment 1 (whereas the positive electrode was notimpregnated with the gelled polymeric electrolyte), and the resultantwas housed in a battery can. Thus, a flat lithium secondary battery(comparative battery) B3 using the gelled polymeric electrolyte wasfabricated.

Discharge Capacity of each Battery at 1st and 100th Cycles

With respect to each of the present batteries A1 through A6 and thecomparative batteries B1 through B3, 100 charge-discharge cycles wererun, in each cycle of which the battery was charged to 4.2 V at acurrent density of 100 μA/cm² and discharged to 2.75 V at a currentdensity of 100 μA/cm² at 25° C., thereby obtaining the dischargecapacity (mAh) of the battery at the 1st and 100th cycles. The resultsare shown in Table 1.

TABLE 1 Discharge capacity (mAh) 1st cycle 100th cycle Present batteryA1 2.2 1.8 Present battery A2 2.2 2.0 Present battery A3 2.1 1.9 Presentbattery A4 2.1 1.9 Present battery A5 2.2 2.0 Present battery A6 2.2 2.0Comparative battery B1 2.2 1.2 Comparative battery B2 2.2 1.2Comparative battery B3 1.9 1.7

As is shown in Table 1, the discharge capacity at the 100th cycle islarger in the present batteries A1 through A6 than in the comparativebatteries B1 and B2. Also, the discharge capacity at the 1st cycle islarger in the present batteries A1 through A6 than in the comparativebattery B3. It is understood from the results that the inventionprovides a lithium secondary battery using a gelled polymericelectrolyte with large initial discharge capacity and goodcharge-discharge cycle performance. The discharge capacity at the 100thcycle of the comparative battery B1 was small probably because manganesewas eluted into the liquid electrolyte, and the discharge capacity atthe 100th cycle of the comparative battery B2 was small probably becausemanganese was eluted into the liquid electrolyte impregnated into thegelled polymeric electrolyte film also serving as the separator.Furthermore, the discharge capacity at the 1st cycle of the comparativebattery B3 is small probably because a contact area between the positiveelectrode and the electrolyte was so small that the contact resistanceon the interface was large.

Embodiment 7

Preparation of Positive Electrode

Lithium nitrate and manganese dioxide were mixed in a molar ratio of1:2, and the mixture was baked at 600° C. for 24 hours in the air,thereby obtaining LiMn₂O₄. The LiMn₂O₄ was crushed with a jet mill togive a spinel type lithium manganese oxide powder with a median diameterof 10 μm. The spinel type lithium manganese oxide powder, a carbonpowder serving as a conductive agent, a poly(vinylidene fluoride) powderserving as a binder and a block copolymer of poly(ethylene oxide) andpolystyrene (with a copolymerization ratio of 1:1 and a number averagemolecular weight Mn of approximately 300,000) were mixed in a weightratio of 85:10:3:2. The resultant mixture was kneaded with NMP to give apaste, the paste was applied on a positive electrode collector (aluminumplate) by the doctor blade method (to attain a thickness of 80 μm afterdrying), and the resultant was heated at 130° C. Thus, a positiveelectrode (porous electrode) in the shape of a disk with a diameter of10 mm was prepared. The positive electrode was immersed in a liquidelectrolyte obtained by dissolving 1 M of LiN(C₂F₅SO₂)₂ in a mixedsolvent including ethylene carbonate and diethyl carbonate in a volumeratio of 1:1, so as to impregnate the block copolymer of poly(ethyleneoxide) and polystyrene with the liquid electrolyte.

Preparation of Gelled Polymeric Electrolyte also Serving as Separator

A block copolymer of poly(ethylene oxide) and polystyrene (with acopolymerization ratio of 1:1 and a number average molecular weight Mnof approximately 300,000) was dissolved in NMP to give a 20 wt %solution. The solution was formed by the casting method into a blockcopolymer film of poly(ethylene oxide) and polystyrene with a thicknessof 25 μm. Next, the film was immersed in a liquid electrolyte obtainedby dissolving 1 M of LiN(C₂F₅SO₂)₂ in a mixed solvent including ethylenecarbonate and diethyl carbonate in a volume ratio of 1:1, so as toimpregnate the block copolymer of poly(ethylene oxide) and polystyrenewith the liquid electrolyte. Thus, a gelled polymeric electrolyte filmalso serving as a separator was prepared.

Fabrication of Lithium Secondary Battery using Gelled PolymericElectrolyte

The gelled polymeric electrolyte film also serving as a separator wassandwiched between the positive electrode and a negative electrode (thesame as that prepared in Embodiment 1) and the resultant was housed in abattery can. Thus, a flat lithium secondary battery (present battery) A7using the gelled polymeric electrolyte was fabricated.

Embodiment 8

A flat lithium secondary battery (present battery) A8 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 1 M ofLiN(CF₃SO₂)₂ in a mixed solvent including ethylene carbonate and diethylcarbonate in a volume ratio of 1:1 was used as the liquid electrolytefor impregnating the block copolymer of poly(ethylene oxide) andpolystyrene in the preparation of the positive electrode and the gelledpolymeric electrolyte film also serving as the separator.

Embodiment 9

A flat lithium secondary battery (present battery) A9 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 1 M ofLiN(CF₃SO₃)(C₄F₉SO₂) in a mixed solvent including ethylene carbonate anddiethyl carbonate in a volume ratio of 1:1 was used as the liquidelectrolyte for impregnating the block copolymer of poly(ethylene oxide)and polystyrene in the preparation of the positive electrode and thegelled polymeric electrolyte film also serving as the separator.

Embodiment 10

A flat lithium secondary battery (present battery) A10 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 1 M ofLiN(C₄F₉SO₂)₂ in a mixed solvent including ethylene carbonate anddiethyl carbonate in a volume ratio of 1:1 was used as the liquidelectrolyte for impregnating the block copolymer of poly(ethylene oxide)and polystyrene in the preparation of the positive electrode and thegelled polymeric electrolyte film also serving as the separator.

Embodiment 11

A flat lithium secondary battery (present battery) A11 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 1 M ofLiC(CF₃SO₂)₃ in a mixed solvent including ethylene carbonate and diethylcarbonate in a volume ratio of 1:1 was used as the liquid electrolytefor impregnating the block copolymer of poly(ethylene oxide) andpolystyrene in the preparation of the positive electrode and the gelledpolymeric electrolyte film also serving as the separator.

Embodiment 12

A flat lithium secondary battery (present battery) A12 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 1 M ofLiC(CF₃SO₂)₂(C₄F₉SO₂) in a mixed solvent including ethylene carbonateand diethyl carbonate in a volume ratio of 1:1 was used as the liquidelectrolyte for impregnating the block copolymer of poly(ethylene oxide)and polystyrene in the preparation of the positive electrode and thegelled polymeric electrolyte film also serving as the separator.

Embodiment 13

A flat lithium secondary battery (present battery) A13 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 1 M ofLiC(CF₃SO₂)(C₄F₉SO₂)₂ in a mixed solvent including ethylene carbonateand diethyl carbonate in a volume ratio of 1:1 was used as the liquidelectrolyte for impregnating the block copolymer of poly(ethylene oxide)and polystyrene in the preparation of the positive electrode and thegelled polymeric electrolyte film also serving as the separator.

Embodiment 4

A flat lithium secondary battery (present battery) A14 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 1 M ofLiC(C₄F₉SO₂)₃ in a mixed solvent including ethylene carbonate anddiethyl carbonate in a volume ratio of 1:1 was used as the liquidelectrolyte for impregnating the block copolymer of poly(ethylene oxide)and polystyrene in the preparation of the positive electrode and thegelled polymeric electrolyte film also serving as the separator.

Embodiment 15

A flat lithium secondary battery (present battery) A15 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 1 M ofLiN(CF₃SO₂)(C₅F₁₁SO₂) in a mixed solvent including ethylene carbonateand diethyl carbonate in a volume ratio of 1:1 was used as the liquidelectrolyte for impregnating the block copolymer of poly(ethylene oxide)and polystyrene in the preparation of the positive electrode and thegelled polymeric electrolyte film also serving as the separator.

Embodiment 16

A flat lithium secondary battery (present battery) A16 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 0.1 M of LiPF₆and 0.9 M of LiN(C₂F₅SO₂)₂ in a mixed solvent including ethylenecarbonate and diethyl carbonate in a volume ratio of 1:1 was used as theliquid electrolyte for impregnating the block copolymer of poly(ethyleneoxide) and polystyrene in the preparation of the positive electrode andthe gelled polymeric electrolyte film also serving as the separator.

Embodiment 17

A flat lithium secondary battery (present battery) A17 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 0.5 M of LIPF₆and 0.5 M of LiN(C₂F₅SO₂)₂ in a mixed solvent including ethylenecarbonate and diethyl carbonate in a volume ratio of 1:1 was used as theliquid electrolyte for impregnating the block copolymer of poly(ethyleneoxide) and polystyrene in the preparation of the positive electrode andthe gelled polymeric electrolyte film also serving as the separator.

Embodiment 18

A flat lithium secondary battery (present battery) A18 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 0.9 M of LiPF₆and 0.1 M of LiN(C₂F₅SO₂)₂ in a mixed solvent including ethylenecarbonate and diethyl carbonate in a volume ratio of 1:1 was used as theliquid electrolyte for impregnating the block copolymer of poly(ethyleneoxide) and polystyrene in the preparation of the positive electrode andthe gelled polymeric electrolyte film also serving as the separator.

Embodiment 19

A flat lithium secondary battery (present battery) A19 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 1 M of LiClO₄in a mixed solvent including ethylene carbonate and diethyl carbonate ina volume ratio of 1:1 was used as the liquid electrolyte forimpregnating the block copolymer of poly(ethylene oxide) and polystyrenein the preparation of the positive electrode and the gelled polymericelectrolyte film also serving as the separator.

Embodiment 20

A flat lithium secondary battery (present battery) A20 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 1 M of LiBF₄in a mixed solvent including ethylene carbonate and diethyl carbonate ina volume ratio of 1:1 was used as the liquid electrolyte forimpregnating the block copolymer of poly(ethylene oxide) and polystyrenein the preparation of the positive electrode and the gelled polymericelectrolyte film also serving as the separator.

Embodiment 21

A flat lithium secondary battery (present battery) A21 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment7 except that a liquid electrolyte obtained by dissolving 1 M of LiPF₆in a mixed solvent including ethylene carbonate and diethyl carbonate ina volume ratio of 1:1 was used as the liquid electrolyte forimpregnating the block copolymer of poly(ethylene oxide) and polystyrenein the preparation of the positive electrode and the gelled polymericelectrolyte film also serving as the separator.

COMPARATIVE EXAMPLE 4

A flat lithium secondary battery (comparative battery) B4 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment3 except that a liquid electrolyte obtained by dissolving 1 M of LiClO₄in a mixed solvent including ethylene carbonate and diethyl carbonate ina volume ratio of 1:1 and a polyethylene microporous film (separator)with a thickness of 25 μm were used instead of the gelled polymericelectrolyte film also serving as the separator and that the gelledpolymeric electrolyte was not included in the positive electrode.

COMPARATIVE EXAMPLE 5

A flat lithium secondary battery (comparative battery) B5 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment3 except that a liquid electrolyte obtained by dissolving 1 M of LiBF₄in a mixed solvent including ethylene carbonate and diethyl carbonate ina volume ratio of 1:1 and a polyethylene microporous film (separator)with a thickness of 25 μm were used instead of the gelled polymericelectrolyte film also serving as the separator and that the gelledpolymeric electrolyte was not included in the positive electrode.

COMPARATIVE EXAMPLE 6

A flat lithium secondary battery (comparative battery) B6 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment3 except that a liquid electrolyte obtained by dissolving 1 M of LiPF₆in a mixed solvent including ethylene carbonate and diethyl carbonate ina volume ratio of 1:1 and a polyethylene microporous film (separator)with a thickness of 25 μm were used instead of the gelled polymericelectrolyte film also serving as the separator and that the gelledpolymeric electrolyte was not included in the positive electrode.

COMPARATIVE EXAMPLE 7

A flat lithium secondary battery (comparative battery) B7 using a gelledpolymeric electrolyte was fabricated in the same manner as in Embodiment3 except that a liquid electrolyte obtained by dissolving 1 M ofLiN(C₂F₅SO₂)₂ in a mixed solvent including ethylene carbonate anddiethyl carbonate in a volume ratio of 1:1 and a polyethylenemicroporous film (separator) with a thickness of 25 μm were used insteadof the gelled polymeric electrolyte film also serving as the separatorand that the gelled polymeric electrolyte was not included in thepositive electrode.

Discharge Capacity of Battery at 1st and 100th Cycles

With respect to each of the present batteries A7 through A21 and thecomparative batteries B4 through B7, 100 charge-discharge cycles wererun under the aforementioned conditions, thereby obtaining the dischargecapacity (mAh) at the 1st and 100th cycles. The results are shown inTable 2.

TABLE 2 Discharge capacity (mAh) 1st cycle 100th cycle Present batteryA7 2.4 2.2 Present battery A8 2.4 2.2 Present battery A9 2.4 2.1 Presentbattery A10 2.4 2.2 Present battery A11 2.4 2.2 Present battery A12 2.42.1 Present battery A13 2.4 2.2 Present battery A14 2.4 2.2 Presentbattery A15 2.4 2.0 Present battery A16 2.4 2.2 Present battery A17 2.42.0 Present battery A18 2.4 1.9 Present battery A19 2.4 1.7 Presentbattery A20 2.4 1.5 Present battery A21 2.4 1.6 Comparative battery B42.4 1.2 Comparative battery B5 2.4 1.1 Comparative battery B6 2.4 1.2Comparative battery B7 2.4 1.2

As is shown in Table 2, the discharge capacity at the 100th cycle islarger in the present batteries A7 through A21 sing the gelled polymericelectrolyte film also serving as the separator because of a smalleramount of manganese eluted during the charge-discharge cycles than inthe comparative batteries B4 through B7 using the liquid electrolyte.Among the present batteries A7 through A21, the discharge capacity atthe 100th cycle is particularly large in the present batteries A7through A18. Accordingly, it is understood that the solute of the liquidelectrolyte used for the impregnation is preferablyLiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) (wherein m and n independentlyindicate an integer ranging between 1 and 5) orLiC(C_(p)F_(2p+1)SO₂)(C_(q)F_(2q+1)SO₂)(C_(r)F_(2r+1)SO₂) (wherein p, qand r independently indicate an integer ranging between 1 and 5).

Relationship Between Concentration of Liquid Electrolyte andCharge-discharge Cycle Performance

Seven kinds of lithium secondary batteries (present batteries) A22through A28 each using a gelled polymeric electrolyte were fabricated inthe same manner as in Embodiment 7 except that a liquid electrolyteobtained by dissolving 0.05 M, 0.1 M, 0.5 M, 1.5 M, 2.0 M, 2.5 M or 3.0M of LiN(C₂F₅SO₂)₂ in a mixed solvent including ethylene carbonate anddiethyl carbonate in a volume ratio of 1:1 was used as the liquidelectrolyte for impregnating the block copolymer of poly(ethylene oxide)and polystyrene in the fabrication of the positive electrode and thegelled polymeric electrolyte film also serving as the separator.Subsequently, with respect to each of the batteries, 100charge-discharge cycles were run under the aforementioned conditions,thereby obtaining the discharge capacity (mAh) at the 1st and 100thcycles. The results are shown in Table 3. Table 3 also shows thedischarge capacity at the 1st and 100th cycles of the present battery A7listed in Table 2.

TABLE 3 Concentration of LiN(C₂F₅SO₂)₂ in liquid electrolyte Dischargecapacity (mAh) (M) 1st cycle 100th cycle Present battery A22  0.05 2.41.3 Present battery A23 0.1 2.4 1.6 Present battery A24 0.5 2.4 1.9Present battery A7 1.0 2.4 2.2 Present battery A25 1.5 2.4 2.0 Presentbattery A26 2.0 2.4 1.6 Present battery A27 2.5 2.4 1.3 Present batteryA28 3.0 2.4 1.3

It is understood from Table 3 that the liquid electrolyte preferablyincludes 0.1 through 2.0 M of LiN(C₂F₅SO₂)₂. It was also confirmed thata concentration of 0.1 through 2.0 M is preferred in using any of otherliquid electrolytes according to this invention represented byLiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) orLiC(C_(p)F_(2p+1)SO₂)(C_(q)F_(2q+1)SO₂)(C_(r)F_(2r+1)SO₂).

INDUSTRIAL APPLICABILITY

The invention provides a lithium secondary battery using a gelledpolymeric electrolyte with large initial discharge capacity and goodcharge-discharge cycle performance.

What is claimed is:
 1. A lithium secondary battery comprising a positiveelectrode comprising a lithium manganese oxide spinel as the activematerial and a gelled polymeric electrolyte (A); a negative electrode;and a gelled polymeric electrolyte (B) in the shape of a film or sheetserving as a separator, the gelled polymeric electrolyte (A) and thegelled polymeric electrolyte (B) each being made from the same materialwhich is a block copolymer of poly(ethylene oxide) and polystyreneimpregnated with a liquid electrolyte.
 2. The lithium secondary batteryusing a gelled polymeric electrolyte according to claim 1, wherein thelithium manganese oxide spinel is LiM_(x)Mn_(2−x)O₄, wherein M is atleast one element selected from the group consisting of Ni, Al, Mg, Feand Co, and 0≦x≦0.5.
 3. The lithium secondary battery using a gelledpolymeric electrolyte according to claim 1, wherein the liquidelectrolyte includes LiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) (wherein mand n independently indicate an integer ranging between 1 and 5) and/orLiC(C_(p)F_(2p+1)SO₂)(C_(q)F_(2q+1)SO₂)(C_(r)F_(2r+1)SO₂) (wherein p, qand r independently indicate an integer ranging between 1 and 5).
 4. Thelithium secondary battery using a gelled polymeric electrolyte accordingto claim 1, wherein the liquid electrolyte includesLiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) (wherein m and n independentlyindicate an integer ranging between 1 and 5) and/orLiC(C_(p)F_(2p+1)SO₂)(C_(q)F_(2q+1)SO₂)(C_(r)F_(2r+1)SO₂) (wherein p, qand r independently indicate an integer ranging between 1 and 5) in aconcentration of 0.1-2.0 mol/liter.